1. Field of the Invention
The invention relates to an apparatus for processing a half tone image.
2. Related Background Art
FIG. 17 shows a schematic construction of a laser beam printer.
Image data 1 is a digital signal of eight bits and shows light and dark states (256 gradations) of an image. As shown in FIG. 18, there is a linear relation between the light and dark information of the image and the gradations (0 to 255). For the digital image signal, an input density of 256 gradations is converted into an output density of 256 gradations by a density converter 2 on the basis of a lookup table system shown in FIG. 19. FIG. 20 shows the relation between the input density and the output density after conversion. The digital image signal is again converted into the analog signal by a D/A converter 6. The analog signal is compared with a signal of a predetermined period which is generated from a triangular wave generating circuit 8 by a comparator 7 and is pulse width modulated. The pulse width modulated binary image signal is directly supplied to a laser driving circuit 9 and is used as a signal to control on/off times of the light emission of a laser diode 10. A laser beam emitted from the laser diode 10 is scanned in the main scanning direction by a well-known polygon mirror 11 and passes through an f-.theta. lens 12 and is reflected by a reflecting mirror 13. The laser beam is subsequently irradiated onto a photo sensitive drum 14 which is rotating in the direction indicated by an arrow in FIG. 17, so that an electrostatic latent image is formed on the drum 14 as an image carrier.
On the other hand, the photo sensitive drum 14 is uniformly discharged by a pre-exposing device 16 before the laser beam is irradiated. Thereafter, the drum 4 is uniformly charged to the minus polarity by a charging device 15. Subsequently, the laser beam is irradiated onto the drum 14 and the electrostatic latent image corresponding to the image signals is formed on the surface of the drum 14.
Onto the portion of the photo sensitive drum 14 which has been discharged by the laser, a toner having a minus charging characteristic is deposited by a developing device 17 on the basis of a well-known inversion developing method. The portion on which the toner has been deposited is developed. A development image (toner image having minus charges) formed on the drum 14 is transferred onto a copy transfer material (generally, paper) 18 by a copy transfer charging device 19. After completion of the copy transfer, the remaining toner on the surface of the drum 14 is scraped off by a cleaner. The above series of processes are repeated again. A density of a final image on the copy transfer material which is formed by the above processes is determined in a one to one corresponding relation for the value of the digital image data which is generated from the density converter 2. That is, a light emitting time of the laser diode is specified in accordance with the value of the digital image data and a quantity of laser beam which is irradiated onto the drum 14 is decided. Further, an amount of toner which is developed, that is, an image density can be specified by causing a predetermined attenuation in a surface potential of the photo sensitive drum in accordance with the laser light quantity.
As shown in FIG. 21, the relation of the image density to the gradation of the digital image data which is generated is not linear but shows an S-shaped curve. Such S-shaped curve characteristics depend on a non-linearity of the sensitivity characteristic of the photo sensitive drum or the developing characteristic to the surface potential of the photo sensitive drum. The above characteristics are based on the characteristics of the conventional electrophotograph.
However, the relation of the final image density to the output image data shown in FIG. 21 is always unstable due to a difference of manufacturing lots of the photo sensitive drum or developing agents, a mechanical variation of the distance between the developing device and the photo sensitive drum, or an environmental durability dependency of the photo sensitive drum or developing agent. Thus, there is a problem such that the gradation of the reproduced image is not good.
To solve the above problems, therefore, a method disclosed in U.S. Pat. No. 4,888,636 has been considered. According to the above method, density of a toner image formed on the photo sensitive drum is measured and characteristics of the output image data and the image density are changed in accordance with the result of the measurement.
According to U.S. Pat. No. 4,888,636, however, a plurality of tables showing the characteristics of the output image data and the image densities are prepared and a proper table is selected in accordance with the result of the measurement. Thus, there is a problem such that the characteristics of the photo sensitive material do not completely correspond to the data in the table and fine control is difficult.